Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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OPTICAL BENCH FOR LASER DESORPTION/IONIZATION MASS SPECTROMETRY
This application claims priority from U.S. Provisional Patent Application
Serial No. G0/134,071 (Atty. Docket No. 1 GBGG-003200US), filed May 13, 1999,
the
disclosure of which is incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTION
1. Field Of The Invention
The present invention relates to a laser desorption ion source, and more
particularly, to a laser optical bench for use with a laser desorption ion
source that
preferentially shapes a beam from a light source by predominantly focusing the
beam in a
single plane.
2. Description Of The Prior Art
A laser desorption ion source is a device that utilizes the energy inherent in
a focused laser beam to promote the desorption of neutrals and/or ions from
solid or
liquid state matter. In the case of solid matter, materials or samples of
interest are
presented as solid state crystals or thin films upon a sample support
typically referred to
as a probe. For liquid matter, the fluids are introduced as droplets or a fine
spray and may
be desorbed in stream or upon a physical support.
The energy transfer process may proceed through direct thermal or
electronic excitation of the material or through indirect thermal excitation.
If the material
directly absorbs energy from the laser source and heats up via direct thermal
or secondary
thermal changes in response to electronic excitation, the process is known as
laser-
induced thermal desorption (LITD). If the material of interest receives
thermal energy
from neighboring compounds while being a member of a co-crystal or thin film
matrix,
the process is known as matrix-assisted laser desorption (MALD). If the
material or
sample of interest has been physically modified, extracted or amplified by the
probe
surface, or if the probe surface contains integral energy absorbing molecules
capable of
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indirect energy transfer to the sample of interest, the process is known as
surfaced
enhanced laser desorption (SELD).
Should preferential ionization be created for the above described
desorption motifs, then such processes are respectively refereed to as laser
desorption
ionization (LDI), matrix-assisted laser desorption/ionization (MALDI), and
surface
enhanced laser desorptionionization (SELDI).
IR;yll'CIICSS Uf wl~icl~ energy transfer process is used, a laser ~lesorlioo
ion
source primarily consists of a collection of components generally referred to
as a laser
optical bench. Such a laser optical bench is schematically represented in
Figure 1.
Generally, a laser optical bench 10 includes a light source or photon source
11, which is generally a continuous beam or pulsed laser, a beam sputter 12,
photodiode
or other photodetector 13, attenuator 14, lens 15, mirror 16 and target 17,
which is
generally a probe including a sample of material of interest.
If a continuous beam laser is employed as light source 11,
desorptioWionization occurs with a constant duty cycle. If desired, high speed
gating of
the beam is typically achieved by using a shutter, which blocks the beam or a
movable
mirror that directs the beam into a beam dump (not shown). If a pulsed laser
is employed
as light source 1 l, the duty cycle is dependent upon the pulse width and
repetition rate.
High speed gating of the beam is achieved by controlling the pulsing process.
In some situations, the laser optical bench may include a photodetector or
photodiode 13 to measure the energy of the laser source or to detect the
lasing event in
the case of pulsed laser applications. Typically, optical beam sputter 12 is
used to divide
off a small fraction of the incident beam and direct it toward the appropriate
photodetector. If the photodetector is used to measure delivered energy, it is
usually of
the thermal, photo-emissive, or semiconductor detector varieties. If the
photodetector
functions to detect the lasing event of a pulsed laser train, the
photodetector is
preferentially a small surface area semiconductor photodiode, which is capable
of
delivering very fast response times.
The propagated laser beam needs to be processed for the purposes of laser
desorption. Such processing often involves control of laser energy, laser
fluence (laser
energy/unit area), and/or laser irradiance (radiant power/unit area). To
achieve the latter,
a combination of lenses and attenuation devices are often used. Typical laser
energy
attenuation devices include a mechanical iris, a neutral density filter or a
fresnel
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reflectionrefraction device. If a neutral density filter has a gradient of
optical densities
allowing for continuous adjustment of transmitted laser energy, it is referred
to as a
gradient neutral density filter (GNDF).
The ultimate size of the focused laser spot on the target is controlled
through prudent selection of mirrors and lenses. Typically, a design that
optimizes
optical throughput while providing the desired fluence or irradiance dynamic
range is
employed. Additionally, the combination of attenuating and focusing elements
should
optimally create an image whose spatial distribution creates a desorption
locus that
promotes maximum sampling area while maintaining maximum ion extraction
efficiency.
Increasing sampling area has three major advantages, specifically
decreased analysis time, improved sample-to-sample reproducibility, and
increased
analytical sensitivity. The advantage of decreased analysis time is readily
apparent and
generally desirable. If one addresses a greater amount of sample area with
each laser
spot, a given sample region may be completely interrogated in less time than
that required
by approaches that employ smaller laser spots.
Typical sample preparation techniques for the previously noted laser
desorption scheme inherently create solid-state or liquid samples with
appreciable
amounts of heterogeneity and microenvironmental differences. These differences
are
sources of qualitative and quantitative in reproducibility when assaying a
plurality of
identical samples. Although some approaches, such as SELDI, function to
minimize
these effects, statistically significant perturbations may still be observed.
The
employment of large laser probed regions improves reproducibility by
increasing the area
of sample investigated for each laser desorption event, statistically
minimizing the effect
of microheterogeneity.
The means by which target probed areas are enlarged is important with
respect to sample laser irradiance. Generally speaking, sample desorption and
ionization
for the previously identified schemes occur at some threshold irradiance
level.
Furthermore, it is often desirable to have the ability to operate at levels
significantly
higher than threshold. Consequently, a given increase in laser spot area would
require a
concomitant increase in laser radiant power. Such laser radiant power
increases may
result in the need to employ more powerful and expensive laser sources.
Accordingly, so,
a means which increases the target sampling area that does not necessitate
significant
increases in laser radiant power is desired.
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Increased analytical sensitivity is achieved by virtue of the fact that more
sample is desorbed and ionized for each desorption event, assuming that the
additional
ionized material within the desorption cloud may be efficiently extracted. The
desorption
cloud can be considered to be a collection of ions, neutrals, and electrons
capable of
shielding externally applied electrical fields. It is generally recognized
that ion extraction
occurs within a given axial length of the desorption cloud known as the plasma
skin
dCptll. T11C plCiSillCl SI<111 ClCptll 1S thalt p0l'tl0l) Of a ClOIlCI'S
OlltCl' pCl'1I11CtC1' f01' w111C17
externally applied electric fields penetrate and do work upon charged
particles. It is
typically determined by the fundamental energetics of the desorption process
and for a
given set of conditions, is considered to be relatively dependent upon the
cloud's charged
particle density.
For the previously noted techniques, desorption cloud charge particle
density has been determined to be dependent upon applied laser irradiance. Low
irradiance levels produce clouds of nominal charged particle density. Under
these
conditions, the plasma skin depth can extend appreciably into the center of
the desorption
cloud and a vast majority of the desorbed ions can be efficiently extracted.
In contrast,
the application of high irradiance levels create clouds of extreme charged
particle density,
producing a plasma skin depth that is a fraction of the total cloud size, thus
providing for
sub-optimal levels of ion extraction. The distinction of low versus high laser
irradiance
levels is dependent upon the ionization technique. For the applications of
SELDI and
MALDI, high laser irradiance can be considered to be that which exceeds
10 mW/cm2.
From the previous explanation, it becomes clear that optimum ion
extraction efficiency will be achieved under conditions for which a maximum
population
of desorbed ions reside within the plasma skin depth. In this manner, laser
spot
geometries that promote desorption clouds with maximized surface area to
volume ratios
are favored.
Further complicating this process is the requirement for creating
homogeneous energy, fluence, or irradiance profiles across the laser spot.
During the
process of desorption and ionization, the initial energy conditions of these
gaseous
products have been shown to be somewhat dependent upon the initial amount of
applied
laser energy. If the laser image contains positional dependent energy
gradients or hot
regions, desorbed products from different regions may exhibit significantly
different
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initial energies. This condition may be detrimental to mass analysis,
especially if non-
orthogonal time-of flight mass spectrometric techniques are employed.
SUMMARY OF THE INVENTION
A laser optical bench, in accordance with the present invention, for use
with a laser desorption/ionization mass spectrometer addresses the
shortcomings of the
prior art. Such a laser optical bench includes a laser' for producing light, a
bealll
expanding focusing structure that receives light from the laser and focuses it
in
predominantly a single plane, an attenuator that receives light from the beam
expanding
focusing structure, a beam steering structure for directing light from the
attenuator to a
target, and an omnidirectional focusing element for focusing light from the
beam steering
stmcture on the target.
The combined action of the aforementioned elements generally serves the
purpose of minimizing laser spot energy heterogeneity while creating a target
probe
sampling spot geometry of enlarged surface area and a desorption cloud with
maximized
surface area to volume ratio.
In accordance with further preferred aspects of the present invention, the
beam expanding focusing structure consists of a pair of cylindrical lenses,
and the laser
optical bench further includes a plano convex lens that focuses the light from
the beam
steering structure onto the target probe.
In accordance with another preferred aspect of the present invention, the
first cylindrical lens of the beam expanding focusing structure preferentially
focuses the
laser beam in a single plane with respect to a gradient neutral density filter
attenuator.
The orientation of the focusing plane is aligned with the gradient direction
of the neutral
density filter so that a minimum energy gradient exists across the beam
transmitted
through the f Iter. Furthermore, because the incident beam is allowed to
diverge in
regions outside of the focusing plane, the laser spot area incident to the
GNDF is
sufficiently large so as to limit the incident irradiance to levels below that
of the GNDF
damage threshold. A second cylindrical lens is used to collect the transmitted
beam and,
in combination with the inherent beam divergence of the laser source, expand
it to match
the numerical aperture of the remaining optical elements.
In accordance with another preferred aspect of the present invention, the
beam steering structure generally includes a mirror that reflects light to a
dichroic filter.
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The dichroic filter allowing some light to pass therethrough while reflecting
a majority of
the light to the target probe. The light transmitted through the dichroic f
Iter is then
preferably passed to a plano convex lens that focuses the light onto a
photodetector in
order to measure the amount of applied laser energy.
Thus, the present invention provides a laser optical bench for use with a
laser desorptionionization mass spectrometer that allows for beam shaping,
which is
crcOcd by prcCcrcOially focusing tl~c laser beam to a minimum dispersion in
only one
plane. By initially focusing the laser beam in a single plane, a decreased
spatial laser
energy gradient across the beam after it passes through the attenuator is
realized.
Furthermore, beam expansion is realized by the combined action of the second
cylindrical
lens and the inherent beam divergence of the laser source, thus utilizing the
full numerical
aperture of the system while selectively allowing expansion in only one
dimension.
Finally, ion desorption loci are created that are shaped in a manner that
optimizes ion
collection/extraction efficiency.
Other features and advantages of the present invention will be understood
upon and reading and understanding the detailed description of the preferred
exemplary
embodiments, found hereinbelow, in conjunction with reference to the drawings
in which
like numerals represent like elements.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic view of a prior as laser optical bench;
Figure 2 is a schematic view of a laser optical bench in accordance with
the present invention;
Figure 3 schematically illustrates a rectangular gradient neutral density
f leer in which the optical density (OD) increases from right to left;
Figure 4 illustrates an improved laser spot on a target probe sample area
created by a laser optical bench in accordance with the present invention; and
Figure 5 is an image of the improved laser spot geometry as achieved with
a laser optical bench in accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EXEMPLARY EMBODIMENTS
With reference to Figure 2, a laser optical bench 10a in accordance with a
preferred embodiment of the present invention is illustrated. The laser
optical bench
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includes a light source or photon source 11 a, preferably in the form of a
laser. A first lens
20 is provided for focusing light from the laser onto an attenuator 21. A
second lens 22 is
provided as a focusing element for focusing light from the attenuator to a
beam steering
apparatus. Preferably, the beam steering apparatus includes a mirror 24 and a
filter 25.
In prefen-ed embodiments, the filter consists of a dichroic filter or a
dichroic mirror.
Finally, a final lens 26 is provided as a focusing element for focusing light
on a target 40,
which is generally a sample probe.
In preferred embodiments, a trigger photodetector or photodiode 30 is
provided as a lasing event sensor. Trigger photodiode 30 receives light from
attenuator
21 and thus, attenuator 21 also serves as a beam sputter in such an
embodiment.
Additionally, in preferred embodiments, laser optical bench 10a includes
an energy measuring apparatus 31 that preferably includes a lens 32 that is
used as a
focusing element for focusing light on an energy photodiode or photodetector
33, which
measures the amount of applied laser energy. Energy measuring apparatus 31
receives
light that is transmitted through filter 25.
In another preferred embodiment, energy measuring apparatus 31 contains
a notch or bandwidth filter 34 so that only light within the wavelength range
of source
11 a is transmitted to the surface of photodetector 33.
Preferably, laser 11 a is a pulsed nitrogen laser. Other lasers, either pulsed
or continuous wave, may also be employed. Light emerging from the laser is
focused by
a first cylindrical lens predominantly in a single plane, preferably in a
vertical plane or a
horizontal plane.
With reference to Figure 3, a configuration of the laser optical bench 10a
wherein light is focused in the vertical plane illustrates the lens 20
creating an image that
is somewhat cigar-shaped. This cigar-shaped image 36 is impinged upon
attenuator 21.
In a preferred embodiment, attenuator 21 is a gradient neutral density filter.
In the
embodiment illustrated in Figure 2, the GNDF is shown to be circular. However,
one
skilled in the art will realize that other geometric arrangements such as
polygonal,
rectangular, or square may also be employed.
Depending upon the nature of the optical density gradient of GNDF, cigar-
shaped image 36 is created in a manner so that a minimal energy gradient
exists across
the beam as it is transmitted through the GNDF. Such a process is depicted in
Figure 3.
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Figure 3 illustrates a rectangular GNDF in which the optical density (OD)
increases from right to left. Cigar-shaped laser spot 36 is vertically
arranged such that a
minimum OD gradient exists along its vertical and horizontal axes, thus
minimizing any
positional dependent energy difference within the transmitted light beam.
Furthermore,
because the spot is allowed to diverge in the vertical plane while being
focused in the
horizontal plane, the over all area of spot 36 is sufficiently large as to
diminish the level
of incident irradiance to be below that of the GNDF damage threshold.
In a preferred embodiment that includes trigger photodiode 30 as a lasing
event sensor, a small portion of the beam incident to GNDF 21 (preferably
approximately
4%) is selectively reflected toward trigger photodiode 30, which is preferable
a high
speed photodetector. Light transmitted through GNDF 21 passes through second
lens 22,
which is used to expand the transmitted light beam.
The expanded light beam then encounters beam steering apparatus 23.
Beam steering mirror 24 is used to adjust for minor alterations and beam
locations by
reflecting the expanded light. Preferably, the expanded light is reflected
toward a filter
25. The filter properties are selected so as to reflect the majority of the
incident radiation
toward the target, while preferably transmitting a small fraction of the
incident beam
(preferably less than 10%) toward energy measuring apparatus 31. A portion of
the
transmitted incident light beam that is transmitted through filter 25 may then
be focused
by lens 32 of energy measuring apparatus 31 through bandwidth filter 34 onto
energy
photodetector 33. This is used to measure the amount of applied laser energy.
The output
of energy photodetector 33 may be calibrated in such a manner so as to reflect
the total
amount of energy being delivered to sample probe 40.
Additionally, it is advantageous for filter 25 to transmit visible light from
target or sample probe 40. In this manner, it may be used as a port through
which direct
sample or laser spot viewing may be possible.
Thus, the combination of mirror 24 and filter 25 is used to create a beam
steering apparatus that directs the beam in the appropriate optical plane
necessary to
optimally strike the target probe, thereby compensating for possible
differences in initial
beam position. Final lens 26 is provided as a focusing element to create the
ultimate laser
spot image 41 upon sample probe 40 by focusing the reflected light beam of
filter 25.
Such an improved laser spot is illustrated in Figure 4
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Preferably, lens 20 and lens 22 are either cylindrical lenses or ellipsoidal
mirrors. Final lens 2G is preferably a concave mirror, a piano convex lens, or
a biconvex
lens. In a preferred embodiment, lenses 20 and 22 are cylindrical lenses,
while lenses 26
and 32 are piano convex lenses. In such a preferred embodiment, lens 20
preferably has a
.75 inch diameter, a 25 mm thickness, and an effective focal length (EFL) of
G.70 mm.
Lens 22 preferably has a 1 inch diameter, 4.3G mm thickness and a 75 mm EFL.
Lenses
2G and 32 prcfcrably havc 20 nun diamclcrs, 3 mm lhickncsscs and 70 mm EFLs.
Lcns
sizes and focal lengths are chosen to operate ideally with a given light
source. Lens
materials are selected to be consistent with wavelength and irradiance/energy
requirements. The above dimensions for the lenses are chosen to ideally work
with a
nitrogen source laser (337 nm) possessing a given amount of beam divergence,
and
having pulse energies of 200 microjoules.
In a preferred embodiment, mirror 24 consists of LTV enhanced aluminum
and has dimensions of 25 mm2 by 6 mm. Also, in a preferred embodiment, filter
25 is a
dichroic filter optimized for 15 degrees of incidence, 90% reflection / 8%
transmission at
337 n.m, 80% transmission at 450 nm, and a 1 inch diameter. Once again, the
size and
composition of the mirror and dichroic filter are selected according to the
incident
wavelength, incident irradiance and beam divergence.
The improved laser spot geometry that results from the laser optical bench
in accordance with the present invention preferably creates an image that has
been
measured to be about 1 mm in width and less than 50 microns in height. Thus,
preferably
a width or length or major axis of the image is approximately 20 times greater
than a
height or length or minor axis of the image. However, the ratio may be between
5 to 1
and 20 to 1 but preferably is around 20 to 1.
Figure 5 depicts the measured laser spot image. This laser spot geometry
results in covering a wide region of the sample probe while simultaneously
producing a
cigar-shaped desorption locus. Even though this laser spot is about 5-10 times
wider than
that of conventional approaches, adequate laser fluence for desorption and
ionization is
obtained by focusing only in one plane, thereby minimizing and conserving
total
irradiated area. In this manner, the need for greater input laser energy
levels is avoided,
thereby allowing the employment of small, low cost laser platforms.
Successive desorption loci are overlapped by progressively advancing the
sample in a vertical direction while the laser spot location remains fixed. In
this manner,
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additional regions of~the sample presenting area may be interrogated. Because
the
desorption locus is preferably cigar-shaped, the resulting desorption plume is
spread out
so as to have a maximized surface area to volume ratio.
The laser optical bench in accordance with the present invention has thus
5 demonstrated improved performance in the formation and collection of ions
created by a
laser desorption ion source in the applications of matrix assisted laser
(1CSU1'ptlUll/1O171GatiUll (MALDI) and surface cnhanccd laser
dcsorption/ionization
(SELDI): The laser optical bench in accordance with the present invention
employs a
cylindrical lens beam expander for the purpose of minimizing laser spot energy
10 heterogeneity while creating a sampling spot with large surface area and
maximized
deso>ption cloud surface to volume ratio.
Those skilled in the art will recognize that a laser optical bench in
accordance with the present invention is suitable for use with a laser
desorption/ionization
mass spectrometer that consists of a magnetic sector, electrostatic analyzer,
ion trap,
quadrapole, other rf mass filter-like analyzer, time-of flight, and ion
cyclotron resonance
device. Additionally, a laser optical bench in accordance with the present
invention is
suitable for use with a hybrid device of two of the above devices.
Furthermore, a laser
optical bench in accordance with the present invention, is suitable for use
with a laser
desorptiorl/ionization ion mobility mass spectrometer.
Although the invention has been described with reference to specific
exemplary embodiments, it will appreciated that it is intended to cover all
modifications
and equivalents within the scope of the appended claims.
